Frequency to voltage converter with means for prescribing pulse width against fluctuations

ABSTRACT

In a frequency to voltage converter, the height of pulses produced in response to the respective cycles of the input signal is stabilized by prescribing the pulse levels with reference to ground and by compensating for the temperature dependency of the characteristics of the circuit elements. Furthermore, the pulse width is prescribed by a predetermined number of cycles of a crystal controlled oscillation. A high gain feedback loop including the converter and a voltage controlled crystal oscillator serves to sweep the frequency of the voltage controlled oscillation in linear proportion to the sweep voltage applied to the loop and to phase synchronize the oscillation with respect to a reference oscillation supplied to the loop. The frequency swept oscillation is applicable to a nuclear magnetic resonance analyser without the offset oscillation.

Funaki et al.

U1] 3,824,410 [451 July 16,1974

FREQUENCY TO VOLTAGE CONVERTER WITH MEANS FOR PRESCRIBING PULSE WIDTH AGAINST FLUCTUATIONS inventors: Hidefumi Funaki; Toshiaki Tanaka;

Katsuyoshi Nakajima; Yuichi Kanda, all of Tokyo, Japan [73] Assignee:

Nippon Electric Varian, Ltd.,

Tokyo, Japan Filed:

June 20, 1972 -Appl. No.: 264,449

Foreign Application Priority Data June 21, 1971 Japan 46-44587 Apr. 18,1972 Japan 47-38312 [52] U.S.Cl

References Cited UNITED STATES PATENTS Jania 307/233 3,723,765 3/1973Kautz et al 328/140 X Primary Examiner-Herman Karl Saalbach AssistantExaminer-Siegfried H. Grimm Attorney, Agent, or FirmSandoe, l-lopgood &

Calimafde [5 7] ABSTRACT In a frequency to voltage converter, the heightof pulses produced in response to the respective cycles of the inputsignal is stabilized by prescribing the pulse levels with reference toground and by compensating for the'temperature dependency of thecharacteristics of the circuit elements. Furthermore, the pulse width isprescribed by a predetermined number of cycles of a crystal controlledoscillation. A high gain feedback loop including the converter and avoltage controlled crystal oscillator serves to sweep the frequency ofthe voltage controlled oscillation in linear proportion to the sweepvoltage applied to the loop and to phase synchronize the oscillationwith respect to a reference oscillation supplied to the loop. Thefrequency swept oscillation is applicable to a nuclear magneticresonance analyser without the'offset oscillation.

3 Claims, 24 Drawing Figures Sweep Variable Freq.0sc. lw-l0 PATENYEU JUL1 6 I974 SHEEI 1 0F 3 Offset Variable Freq. Osc. j

I /-I3 2nd. Utitiz. I j MOD. Filter Device 2 l? l'r ls \IQ NE I?) l er i080.

(Prior Art) FIG.|

PIC-3.70

FlG.7b

FIG.7c

FlG.7e

FlG.7f

FlG.7g

FlG.7h

Input 35 Shaped Signal 72 o F/F Output 74 o Inverted I F/F Output osc.

Output 88 0 Count 92 Inverted Count 94 O F/FOutput 1 Reset(74) QInverted l F/F Resumed (7 PATENT BJUUBIBH SW BF 3 3.824.410

M 5 4 2 in v 3 A r .m r m .3 2 m r e p G h S W 2 2 27(E I 29 V FIG.2

Width Presc'd.

Input 22(F') FIG.30

Shaped w F|G.3b

FIG.3c

i Hei mW Presc'd.

27 FlG.3d

Sig.23

(Pnor Art) f Urlllzohon Device Filter I Variable Freq.O5 f Main Mod.

Frequency Converter :H 8| f 0 0 '0 5 mms f0 M00 0 AVG m f 2 r m e D. .4m 8 O of W" C0 0 av i V Neg. Feedback Producer Pulse FREQUENCY TOVOLTAGE CONVERTER WITH MEANS FOR PRESCRIBING PULSE WIDTH AGAINSTFLUCTUATIONS BACKGROUND OF THE INVENTION ing the frequency of theelectric oscillation produced by a stable voltage controlled oscillatordisposed in the loop by an adjustable sweep voltage supplied to such aloop. The sweep so effected on a high frequency sinusoidal oscillationis of prime importance to nuclear magnetic resonance analysis.

A simple variable frequency oscillator is known which comprises either aparallel or a series circuit of a crystal resonator and a variablecapacity diode supplied with a control D. C. voltage for sweeping thefrequency of the oscillator output. This oscillator is unsatisfactory incase an exact rectilinear relationship between the applied sweep voltageand the frequency swept thereby is indispensable.

Another conventional variable frequency oscillator device, which willlater be described with reference to FIG. I, is also objectionable inthat the device is incapable of providing a stable, reproducible, anddistortionless output oscillation linearly and quickly variable over awide range of sweep.

On the other hand, a frequency to voltage converter is known. such aswill hereinafter be described with reference to FIG. 2. The converter,however, has but a poor linear relationship between the frequency of theinput signal and the voltage of the output signal.

SUMMARY OF THE INVENTION It is therefore an object of the presentinvention to provide a frequency to voltage converter having the bestpossible linearity between the input frequency and the output voltage.

Another object isto provide a variable frequency oscillator devicehaving the best possible frequency stability.

Still another object is to provide a variable frequency oscillatordevice having the best possible linearity between the applied sweepvoltage and the output frequency swept thereby.

Yet another object is to provide a variable frequency oscillator devicehaving the best possible reproducibility of the output frequency.

A further object is to provide a variable frequency oscillator devicehaving a wide range of frequency sweep.

A still further object is to provide a variable frequency oscillatordevice whose output frequency can be swept at a high speed.

A further object is to provide a variable frequency oscillator capableof producing the purest possible sinusoidal oscillation.

A subordinate object is to provide a variable frequency oscillator foruse in operating a nuclear magnetic resonance analyser without an offsetoscillator.

According to the instant invention there is provided a converter forconverting the frequency of an input signal to the voltage of an outputsignal including cir- LII cuit means responsive to said input signal forderiving a train of pulses having an approximately predetermined width,wherein the improvement comprises means for prescribing said widthwithin a tolerance of about 1 X 10 times the intended width.

According to an aspect of this invention there is provided a converterof the type described, wherein said pulse width prescribing meanscomprises first means for prescribing the levels of said pulses withreference to the ground of a power source for said circuit means andsecond means'for compensating for the temperature dependency ofthecharacteristics of said circuit means.

According to another aspect of this invention there is provided aconverter of the type described above, wherein said pulse widthprescribing means comprises a crystal oscillator for producing a sharplyrising oscillation and means for starting said oscillation when saidinput signal assumes a predetermined level and stopping said oscillationwhen apredetermined number of cycles of said oscillation have occurred.

According to still another aspect of this invention there is provided anadjustable variable frequency oscillator comprising a voltage controlledstable oscillator, a high gain negative feedback loop therefor, areference frequency oscillator, and a source for producing an adjustablesweep voltage, said loop in turn comprising a frequency converterresponsive to the outputs of said voltage controlled and said referencefrequency oscillators for producing a signal of the frequency determinedby the frequencies of the last-mentioned outputs, a frequency to voltageconverter of the type described for producing in response to thelast-mentioned signal an output signal of the voltage determined by thelast-mentioned frequency, and means responsive to the last-mentionedvoltage and said sweep voltage for controlling the frequency of theoutput of said voltage controlled oscillator.

BRIEF DESCRIPTION OF THE DRAWINGS For a more detailed understanding ofthe invention, reference may be made to the description below taken inconjunction with the drawings wherein:

FIG. 1 is a block diagram of a conventional variable frequencyoscillator;

FIG. 2 is a block diagram of a conventional frequency-to-voltageconverter;

FIG. 3a shows the wave form of a signal supplied to the converterillustrated in FIG. 2;

FIG. 3b shows the wave form of the output signal of a shaper used in theconverter;

FIG. 30 shows the width prescribed signal produced by a pulse widthprescriber used in the converter;

FIG. 3d shows the height prescribed signal produced by an amplifier usedin the converter;

FIG. 4 is a schematic circuit diagram of a variable frequency oscillatorincluding a first embodiment of the present invention;

FIGS. 5a through 5g show wave forms appearing at various points in thefirst embodiment;

FIG. 6 is a schematic circuit diagram of a variable frequency oscillatorincluding a second embodiment of this invention;

FIG. 7a shows the wave form of a signal supplied to the secondembodiment of this invention used in the variable frequency oscillatorillustrated in FIG. 6;

FIG. 7b shows the wave form of the signal produced by an input NANDgateused in the second embodiment;

FIG. 7c shows the wave form of the output signal of a flip-flop circuitresponsive to the signal illustrated in FIG. 7b;

FIG. 7d shows the wave form obtained by inversion of the signal depictedin FIG. 70;

FIG. 76 shows the wave form of the oscillation produced by an oscillatorused in the second embodiment;

FIG. 7f shows the wave form of the signal produced by a cycle counterfor countingthe cycles of the oscillation;

FIG. 7g shows the wave form derived by inversion of the signalillustrated in FIG. 7f;

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1, aconventional variable frequency oscillator mentioned in the preamble ofthis specification and used in a frequency sweeper device 10 for anuclear magnetic resonance analyser will now be described in some detailin order to facilitate the understanding of the present invention. Thefrequency sweeper device comprises a high frequency oscillator 11 forgenerating an electric oscillation of a frequency F and a sweep voltagesource 12 for producing an adjustable sweep D. C. voltage v. Thefrequency. F may be 15 MHz. The sweep voltage source 12 may be apotentiometer coupled with the X-axis drive for the recorder of theanalyser. The device further comprises a low frequency oscillator 13controlled by thesweep voltage v for producing an electric oscillationofa varying frequencyf. a first modulator 14 supplied with the high andthe varying'frequency oscillations for delivering a first modulationoutput having an upper side band component of the frequency F f and alower side band component of the frequency F f. The modulator 14 ispreferably a balanced modulator. The device still further comprises a'first filter 15 having a pass band for the desired side band component.The varying frequencyfis of the order of SkHz. It is therefore necessarythat the filter 15 be capable of sufficiently attenuating the unwantedside band component which is spaced apart from the desired side band byonly kHz. It is further desirable that the filter have a wide pass bandwidth to provide a wide range of sweep. These requirements for thefilter 15 are somewhat contradictory, with the result that it isdifficult for the filter 15 to derive the desired single side bandcomponent to provide a purest possible sinusoidal oscillation asrequired by the nuclear magnetic resonance analysis. The purity might beraised if the frequency difference between the side band components werestressed by raising the varying frequency f. This naturally widens therange of the frequency sweep but deteriorates the stability of thevarying frequency f.

Referring further to FIG. 1, the frequency sweeper 10 still furthercomprises an offset oscillator 16 for generating an electric oscillationof a relatively low frequency f, a second modulator 17 supplied with theoutputs of the first filter l5 and the offset oscillator 16 fordelivering a second modulation output again having upper and the lowerside band components, and a second filter 18 having a pass band whichincludes the preferred component. Inasmuch as the output of the firstfilter 15 includes the unwanted components and consequently hasfrequencies F F f, and F f, the second modulation output has frequenciesF O i f, F 0 f if, and F f i f. The off-set frequency j should beadjustable over a range of approximately kHz in view of the possiblerange of the chemical shifts of various atomic nuclei. Accordingly, thesecond filter 18 should have a pass band width of about 30 kHz, whichcovers the frequencies of at least three components of the secondmodulation output under the present circumstances where it is necessaryto use a sweep frequency f of the order of SkHz. It isthereforeunavoidablethat the high frequency oscillation obtained from the secondfilter 18 having unwanted frequency components is supplied to autilization device 19 in the analyser. In addition, the third orderdistortion inevitably resulting from the second modulator 17 furtherdeteriorates the purity of the high frequency oscillation. This bringsabout beats and other problems in the nuclear magnetic resonance.

Referring to FIGS. 2 and 3, a conventional frequency-to-voltageconverter referred to in the preamble will be described for conveniencein describing'the instant invention. The converter comprises a shaper 21supplied with an input signal 22 having a frequency F for producing ashaped rectangular signal 23. FIGS. 3a and b illustrate, by way ofexample, the input signal 22 as a sinusoidal oscillation and the shapedrectangular signal 23, respectively. The frequency F may either be ahigh frequency or a relatively low frequency. The converter furthercomprises a pulse width prescribing circuit 24 often comprising adifferentiation circuit and a monostable multivibrator for deriving atrain 25 of pulses having an approximately constant pulse width Wdepicted in FIG. 30, an amplifier 26 for amplifying the pulse train 25to another train 27 of pulses of ajpredetermined height E shown in FIG.3 d, and an integrator 28 for integrating the amplified pulsetrain 27 toderive an output signal 29 whose level V is approximately linearlyproportional to the input frequency F. More particularly, the outputvoltage V is given by where T represents the period of the input signal22, while the output voltage V is sufficiently smaller than the pulsevoltage E..If the input frequency F remains unchanged, the outputvoltage V is dependent on the pulse width Wand the pulse height E.'Theconventional converter has but a poor'linearity between the inputfrequency F and the output voltage V because of the fluctuations in thepulse width and height.

Referring to FIG. 4, a frequency sweeper device 10 for a nuclearmagnetic resonance analyser including a frequency-to-voltage converterof a first embodiment of the present invention comprises a highfrequency oscillator 11, a sweep voltage source 12, and an offsetoscillator 16, all similar to the corresponding circuit componentsillustrated in FIG. 1. The device further comprises a main modulator 17and a filter 18 and is accompanied by a utilization device 19, which arethe equivalents of the second modulator and filter l7 and 18 and theutilization device 19, respectively. Preferably, the high frequencyoscillator 11 is a crystal oscillator for generating a stable electricoscillation of the reference frequency F The device still furthercomprises a negative feedback loop 31 which in turn comprises a voltagecontrolled crystal oscillator 33 for producing an electric oscillationof the varying high frequency F a frequency converter 34, such as abalanced modulator, supplied with the oscillations of the reference andthe varying frequencies F and F, for deriving the modulation output 35,a frequency to voltage converter 36 for converting the frequency F(which is equal to F F, in this case) of the modulation output 35 to afeedback D. C. voltage V of the polarity opposite to the adjustablesweep D. C. voltage v, and a comparator 38 having a D. C. amplifier andsupplied with the sweep and the feedback voltages v and V for producingan error signal 39 representative of that difference between such D. C.voltages which is always urged towards zero. When the absolute value ofthe feedback voltage V is equal to that ofa givensweep voltage v, thevarying frequency F is kept unchanged by the loop 31,

, whereas a change AF if produced for one reason or other, will resultin a change AV of the feedback voltage V as given by Equation 1:

AV= E-WA(F F,) EW-AF,

for producing a corresponding error signal 39, which acts on the voltagecontrolled oscillator 33 to restore the original varying frequency FWhen the sweep voltage v varies by an amount Av, a corresponding errorsignal 39 is produced to change the varying frequency F by acorresponding amount AF given by:

thereby returning the error to zero. The varying frequency F is thuslinearly controlled by the sweep voltage i'. It is to be noted here thatthe frequency F of the signal 35 supplied to the frequency-to-voltageconverter 36 is.significantly lower than the reference and varyingfrequencies F and F,, with the result that it is easy to separate thevarious components ofthe modulation output 35 by frequency. Thedifference frequency F may be about kHz kHz.

Further referring to FIG. 4, the oscillation of the varying frequency F,is directly led to the main modulator 17. This obviates the distortionwhich would otherwise be introduced into the oscillation by nonlinearcircuit components, such as the first modulator 14. If the frequency tovoltage converter 36 is stable enough and if the loop 31 has asufficiently high gain, the varying frequency oscillation produced bythe voltage controlled oscillator 33 is substantially phase synchronizedwith the reference oscillation generated by the high frequencyoscillator 11. As a result, the negative feedback loop 31 may be lookedupon as a phase synchronizing loop. This assures the purity and thestability of the varying frequency oscillation. If thefrequency-tovoltage conversion is sufficiently linear, the high gainmakes it additionally possible to improve the linearity between thevariation in the sweep voltage v and the variation in the varyingfrequency F to a level satisfactory for magnetic resonance analysis.More particularly, a loop gain of,8 restricts the deviation from theexact rectilinearity to H3. For example, a gain of 1,000 is sufficient,which limits the deviation to about 0.1 percent. A higher gain is of nopractical use because this linearity is comparable with the present dayprecision of the sweep voltage v. The stability of the outputoscillation and the linearity between the sweep voltage v and thevarying frequency F 1 results in a high reproducibility for the varyingfrequency F Referring still further to FIG. 4 and additionally to FIG.5, a frequency-to-voltage converter 36, which may be used for variouspurposes other than magnetic resonance analysis, comprises a pulseproducer 41 supplied with an input signal 35 of a frequency F depictedin FIG. 50 for producing a train 42 of pulses shown in FIG. 5b, abistable multivibrator 43 responsive to the train 42 for producing ashaped rectangular oscillation 44 shown in FIG. 5c, and an NPN switchingtransistor 45 supplied with the rectangular oscillation 44. Dependingupon the intended use of the frequency to voltage converter 36, theinput frequency F may either be high or low. In addition, the inputsignal 35 need not be sinusoidal. In order to provide an output D. C.voltage V which has the best possible linear relationship with the inputfrequency F, a rectangular voltage 46 developed at the collector of thetransistor 45 should be devoid of effects attributable to thetemperature dependency of the saturation level of the transistor 45, thefluctuation of the voltage supplied from a power source 49, and othercauses. According to this invention, a first Zener diode 50 isinterposed together with a resistor between the collector and the powersource 49 so that the Zener voltage V prescribes the voltage of theswitched rectangular voltage 46, which is supplied to a differentiationcircuit 51 including a capacitor 52 and a resistor 53. Again, theeffects of the fluctuation of the source voltage should-be excluded fromthe differentiated output 54. Further according to this invention, thisexclusion is achieved by leading the differentiated output 54 to a PNPtransistor 56 ordinarily operable at the saturation level, with theemitter connected to a power source 49. In this manner, the electricpotentials on both sides of the capacitor 52 are linked with thepotential provided by the power source 49 so as not to vary relative tothe latter potential, however the latter may fluctuate. Furthermore, asecond Zener diode 57 is connected across the resistor 53 to link thevoltage appearing thereacross with the source potential.

Further referring to FIGS. 4 and 5, the switched rectangular voltage 46and the differentiated output 54 thus assume the wave form illustratedin FIGS. 5d and e, respectively. The low input impedance of the base ofthe PNP transistor 56 additionally serves to deprive the differentiatedoutput 54 of the negative going pulses which would otherwise appear,occurring at times T T and so on shown in FIGS. 5d and e. The pulsewidth W of the differentiated output 54 is given by where RC is the timeconstant of the differentiation circuit 51 and the quantity V is givenby where in turn V is the Zener voltage, of the second Zener diode 57and V is the baseemitter voltage of the PNP transistor 56. Among thevariables in the right side of Equation (2), the Zener voltages V and Vare not variable with the source voltage. The capacity C and theresistance R are also free from fluctuations because capacitors andresistors having excellent temperature characteristics are readilyobtained. The baseemitter voltage V however, is subject to a temperaturedependency of about 2.5mV/C. It is therefore necessary in order to-makethe temperature dependency of the pulse width W negligible that thesecond Zener voltage V be considerably larger than the baseemittervoltage V More particularly, let the ratio VRE/Vpg be represented by .r.The rate Ax of change of quantity V is given by A): AVng/Vpg z AV V 2 X10 V 2 C On the other hand, the pulse width W given by Equation (2) isnow given by W= R-C-ln[l (l -V,,,/V,, R'C'lnU (l +1 +3 .)'V1 g/V IV[)]/V[)2)'(I in/ iral m/ mI- I U in/ 02) 'U !)1/ l)2) Ih mI' with theresult-that the temperature dependency and the rate thereof are .givenby and W/ W I( m m/ nzl an respectively. When both Zener voltages V andV are about l V, the rate AW/W is approximately equal to 2 X 10* C whichis as small as the temperature dependency of the Zener voltage. Thisrate, however, is still too large because it gives a rate-of l Hz/C tothe input frequency F of the order of SkHz. It is therefore furthernecessary that a silicon diode 59 be interposed in the forward directionbetween the second Zener diode 57 and the power source 49 to compensatefor temperature dependency.

As described in detail hereinabove, it has been found that the circuitryincluding the differentiation circuit 51 makes an important improvementin the linearity of the frequency to voltage conversion characteristic.The first embodiment has improved the linearity by providing a voltageprescribing means in the circuitry for making the voltage appearingtherein depend on the potential of the power source 49 and forcompensating for the temperature dependency of the characteristics ofthe circuit elements in the frequency to voltage converter 36. As isknown in the art, the collector of the PNP transistor 56 provides arectangular oscillation 60 illustrated in FIG. f, which is led to athird transistor 61 whose collector output should now be prescribed withreference to ground for the power source 49. This is achieved by anothervoltage prescribing means comprising a third and a fourth Zener diode 63and 64. The collector voltage 65 derived from the third transistor 61thus assumes a wave form depicted in FIG. 53, the

- mean D. C. level giving the output D. C. voltage V shown therein by abroken line. The output voltage V is thus prescribed by the invariablepulse width W and the last-mentioned voltage prescribing means 63 and 64so as to be very stable for a given input frequency F 1 and to vary inthe best possible linear relationship to the input frequency F.

Referring to FIGS. 6 and 7, a frequency sweeper device with afrequency-to-voltage converter of a second embodiment of the instantinvention comprises a reference frequency oscillator 11, a sweep voltagesource 12, an accompanying utilization device 19, an amplifier 26, anintegrator 28, a voltage controlled crystal oscillator 33, a frequencyconverter 34, a

frequency-to-voltage converter 36, and a comparator 38, all beingequivalents of the respective circuit members illustrated in FIGS. 2 and4. The frequency to voltage converter 36 comprises a first NAND gate 71supplied with the modulation output 35 of the frequency P a half cycleof which is depicted in FIG. 7a for producing a shaped rectangularsignal 72 shown in FIG. 7b, a flip-flop 73 put in the reset state in themanner later described and set by the shaped rectangular signal 72 toturn the flip-flop output signal 74 into the logic 0 state illustratedin FIG. 70, a second NAND gate 75 for producing an inverted signal 76shown in FIG. 7d, and a third NAND gate 77 supplied with the invertedsignal 76 at one of its two input terminals, and a fourth two-input NANDgate 78 similarly supplied with the inverted signal 76. The converter 36further comprises a crystal oscillator 81 which in turn comprises thethird and the fourth NAND gates 77- and 78,

i a crystal resonator 82, coupling capacitors 83 and 84,

feedback resistors 85 and 86 shunting the other input terminals and theoutput terminals of the respective two-input NAND gates 77 and 78, andanother capacitor 87 for adjusting the positive feedback for the crystalresonator 82. While the inverted signal 76 is in the logic l state, thecrystal oscillator 81 generates an electric oscillation 88 of thecharacteristic frequency of the crystal resonator 82.- It is possible byproper selection of the circuit constants to make the oscillation 88rise sharply in the manner illustrated in FIG. 72. The converter 36still further comprises a cycle counter 91 responsive to the oscillation88 for producing a logic 0 pulse 92 depicted in FIG. 7f each time thecycles of the oscillation 88 reaches a predetermined number such as, forexample, 10 and a fifth NAND gate 93 responsive to the pulse 92 forsupplying an inverted pulse 94 shown in FIG. 7g to the flip-flop circuit73 to reset the same in the manner depicted in FIG. 7h, thereby stoppingthe oscillation 88 and resetting the cycle counter 91 through aconnection 95 and the output signal 76 of the second NAND gate 75 tologic 0" asshown in FIG. 71'. This operation repeats for every cycle ofthe modulation output 35, with the result that the circuitry describedabove comprises in effect an oscillator 81 and a frequency demultipliertherefor. The frequency demultiplied output 76 is a train of pulsescorresponding to the train 25 shown in FIG. 3c and having such aprescribed width Wwhich is determined by the number of cycles counted bythe cycle counter 91 and is as stable as the characteristic frequency ofthe crystal oscillator 81.

With the second embodiment of this invention, to which variousmodifications are possible, the pulse width W is prescribed with theprecision of the crystal oscillator 81. This provides not only the bestpossible linearity between the input frequency F l and the outputvoltage V but also the best possible stability and reproducibility ofthe varying frequency F As another unexpected result, it is possible toselect a considerably higher frequency F for the modulation output 35,thereby permitting a wide range of the frequency sweep and a high sweepspeed. With 15 MHz selected for the reference frequency F the range ofthe best possible linear sweep may be as wide as I00 kHz or even more.With a sweep range of I00 kHz and with a given sweep voltage v, thevariable frequency F 1 does not vary beyond 0.1 Hz for a considerableduration of time. Ths same parameters apply to the first embodiment.Consequently, the present invention makes it possible to provide afrequency sweeper for a nuclear magnetic resonance analyser having nooffset oscillator 16, no main modulator l7, and no filter l8, avoidingthe unwanted frequency components and the undesirable distortion.

The circuits described above are exemplary and susceptible of variationand modification without departing from the spirit or scope of theinvention. Therefore, the invention is not deemed to be limited to theparticular embodiments that have been described in detail.

What is claimed is:

l. A frequency-to-voltage converter for producing an output signalhaving a voltage corresponding to the frequency of an input signalcomprising:

first means for receiving said input signal and producing a first pulsetrain in accordance with the frequency thereof;

a line for connection to a direct current power supply, and a loadconnected to said line to provide a voltage drop;

second meansconnected to said first means to receive said first pulsetrain and connected to said load to receive said direct current forproducing a second pulse train having an approximately predeterminedwidth;

third means for prescribing the amplitude of said sec- 0nd pulse trainwith reference to the voltage level of said direct current, said thirdmeans comprising a Zener diode connected to said power supply line inparallel with said load;

differentiation circuit means connected to said third means to receivesaid amplitude prescribed second pulse train for producing adifferential output;

fourth means for insulating the converter from the effects oftemperature variations comprising a silicon diode interconnected betweensaid power supply lines and said differentiation circuit means; and

integrator means connected to said differentiation circuit means forintegrating the output thereof. 2. A converter according to claim 1,further comprising fifth means for prescribing the amplitude of saiddifferential output with reference to a ground to insulate saiddifferential output from variations in the voltage of said directcurrent power supply.

3. A converter according to claim 1, wherein: said differentiationcircuit means comprises a resistor and a capacitor, said capacitorhaving a first electrode connected to said load and a second electrodeconnected to said resistor; and said fifth means further comprises anadditional Zener diode connected across said resistor.

1. A frequency-to-voltage converter for producing an output signalhaving a voltage corresponding to the frequency of an input signalcomprising: first means for receiving said input signal and producing afirst pulse train in accordance with the frequency thereof; a line forconnection to a direct current power supply, and a load connected tosaid line to provide a voltage drop; second means connected to saidfirst means to receive said first pulse train and connected to said loadto receive said direct current for producing a second pulse train havingan approximately predetermined width; third means for prescribing theamplitude of said second pulse train with reference to the voltage levelof said direct current, said third means comprising a Zener diodeconnected to said power supply line in parallel with said load;differentiation circuit means connected to said third means to receivesaid amplitude prescribed second pulse train for producing adifferential output; fourth means for insulating the converter from theeffects of temperature variations comprising a silicon diodeinterconnected between said power supply lines and said differentiationcircuit means; and integrator means connected to said differentiationcircuit means for integrating the output thereof.
 2. A converteraccording to claim 1, further comprising fifth means for prescribing theamplitude of said differential output with reference to a ground toinsulate said differential output from variations in the voltage of saiddirect current power supply.
 3. A converter according to claim 1,wherein: said differentiation circuit means comprises a resistor and acapacitor, said capacitor having a first electrode connected to saidload and a second electrode connected to said resistor; and said fifthmeans further comprises an additional Zener diode connected across saidresistor.